1. Field of the Invention
The present inventions relate to preparation of conductive foil substrates for thin film solar cell fabrication, especially solar cells employing Group IBIIIAVIA absorbers.
2. Description of the Related Art
Solar cells are photovoltaic devices that convert sunlight directly into electrical power. The most common solar cell material is silicon, which is in the form of single or polycrystalline wafers. However, the cost of electricity generated using silicon-based solar cells is higher than the cost of electricity generated by the more traditional methods. Therefore, since early 1970's there has been an effort to reduce cost of solar cells for terrestrial use. One way of reducing the cost of solar cells is to develop low-cost thin film growth techniques that can deposit solar-cell-quality absorber materials on large area substrates and to fabricate these devices using high-throughput, low-cost methods.
Group IBIIIAVIA compound semiconductors comprising some of the Group IB (Cu, Ag, Au), Group IIIA (B, Al, Ga, In, Tl) and Group VIA (O, S, Se, Te, Po) materials or elements of the periodic table are excellent absorber materials for thin film solar cell structures. Especially, compounds of Cu, In, Ga, Se and S which are generally referred to as CIGS(S), or Cu(In,Ga)(S,Se)2 or CuIn1-xGax (SySe1-y)k, where 0≦x≦1, 0≦y≦1 and k is approximately 2, have already been employed in solar cell structures that yielded conversion efficiencies approaching 20%. Absorbers containing Group IIIA element Al and/or Group VIA element Te also showed promise. Therefore, in summary, compounds containing: i) Cu from Group IB, ii) at least one of In, Ga, and Al from Group IIIA, and iii) at least one of S, Se, and Te from Group VIA, are of great interest for solar cell applications.
The structure of a conventional foil-substrate based Group IBIIIAVIA compound photovoltaic cell such as a Cu(In,Ga,Al)(S,Se,Te)2 thin film solar cell is shown in FIG. 1. The device 10 is fabricated on a base 10A which includes a substrate 11 coated with a conductive layer 13. The substrate 11 is a conductive foil or web, such as such as stainless steel. Various metallic foil substrates, such as Cu, Ti, Mo, Ni, Al, stainless steel have previously been identified for CIGS(S) solar cell applications (see for example, B. M. Basol et al., “Status of flexible CIS research at ISET”, NASA Document ID:19950014096, accession No: 95N-20512, available from NASA Center for AeroSpace Information, and B. M. Basol et al., “Modules and flexible cells of CuInSe2”, Proceedings of the 23rd Photovoltaic Specialists Conference, 1993, page 426). The absorber film 12, which comprises a material in the family of Cu(In,Ga,Al)(S,Se,Te)2, is grown over the conductive layer 13, which is previously deposited on the substrate 11 and which acts as the electrical contact to the device. Various conductive layers 13 or contact layers comprising contact materials such as Mo, Ta, W, Ti, TiN etc. have been used in the solar cell structure of FIG. 1. The conductive layer 13 may be a single layer or a stacked layer. For example, Cr/Mo stacked conductive layer is commonly used in the CIGS(S) solar cell structure because Cr improves adhesion of Mo to the substrate. After the absorber film 12 is grown, a transparent layer 14 such as a CdS, ZnO, CdS/ZnO stack or CdS/ZnO/ITO stack is formed on the absorber film. Radiation 15 enters the device through the transparent layer 14. Metallic grids (not shown) may also be deposited over the transparent layer 14 to reduce the effective series resistance of the device.
One problem associated with the device structure of FIG. 1 is the surface roughness of the foil substrate 11. As depicted in FIG. 1, the surface of metallic foils such as stainless steel foils often have protrusions 15 and pits 16, which may have heights or depths in the range of 500-2000 nm, or even larger. The contact layers or conductive layer 13 is typically a 200-1000 nm thick film of a refractory metal such as Mo, typically deposited by a physical vapor deposition technique such as sputtering. Therefore, when deposited over the rough surface of the substrate 11, the conductive layer 13 may have discontinuities, especially over the locations of the protrusions 15 and pits 16. When the absorber film 12 is formed over the contact layer 13, the substrate 11 would be exposed to the absorber film 12 at the locations of the discontinuities. As a result, chemical interaction may take place between the substrate 11 and the absorber film 12 at these locations. For example, for stainless steel substrates, Fe diffusing from the substrate 11 into the absorber 12 through the discontinuities in the contact layer 13 may poison the regions of the absorber 12 directly above the discontinuities. When the solar cells are completed by deposition of the transparent layer 14, these poisoned regions cause excessive current leakage reducing the efficiency of the devices. Alternately, the regions where the contact layer 13 is discontinuous, don't get coated effectively by the absorber film 12, giving rise to pinholes, which are very undesirable defects for solar cells as they render the cells inoperable.
In the past researchers have deposited thick insulating layers on foil substrates to planarize and at the same time insulate the surface of such metallic foils. FIG. 2 shows such a structure where the rough surface 20 of a metallic foil substrate 21 is coated with a planarizing insulator 22. The thickness of the planarizing insulator 22 is larger than the roughness of the rough surface 20, therefore, it provides a smooth surface 22A on which a solar cell may be fabricated. Fabrication of the solar cell includes the steps of; i) depositing a conductive contact 23 on the smooth surface 22A, ii) depositing an absorber layer 24 on the conductive contact 23, and iii) forming a transparent conductive top contact 25 on the absorber layer 24. It should be noted that, in the structure of FIG. 2, the metallic foil substrate 21 is electrically isolated from the conductive contact 23 by the planarizing insulator 22. Therefore, the two terminals of the device are connected to the conductive contact 23 and the transparent conductive top contact 23.
Since the current passes through the relatively thin conductive contact, which may be a 100-1000 nm thick metal such as Mo, the device structure of FIG. 2 cannot be used to fabricate large cells. If large devices with at least one dimension larger than 5-20 cm are fabricated using this approach, the excessive voltage drop within the thin conductive contact would deteriorate the fill factor values and reduce conversion efficiency.
This is the reason structures such as the one shown in FIG. 2 may be used for making thin film cell modules employing well known monolithic integration techniques where narrow but long solar cells are interconnected in series on top of the thick insulating layer. Such solar cells may have a width of 0.5-5 cm and a length of 30-60 cm or even longer. As described before, once the width of the cells become much larger than about 5 cm, voltage drop causes deterioration of the solar cell parameters, especially fill factor.